In an optical disk device using an optical disk as a recording medium an optical pickup is used for reading out information pits, by means of which information signals are written-in on the disk. In such an optical pickup it is necessary to focus precisely a laser beam to these information pits on the disk. For this purpose, an objective lens driving device is used, which moves an objective lens two-dimensionally in the direction of the optical axis, i.e. according to a focusing signal, and in the tracking direction against deviations or oscillation of the disk. Various properties required to this objective lens driving device are: (1) to have a high sensitivity; (2) to be excellent in dynamic characteristics such as the ability of following in a high frequency region; (3) to be of small size; (4) to be cheap, etc.
FIGS. 16 to 22 illustrate examples of various sorts of prior art objective lens driving devices.
In the example indicated in FIG. 16, an objective lens 12 is mounted on a lens holder 11A, which is rotatable around a journal 16 and movable in the direction of the journal 16 and a driving coil for focusing 15b is wound in the form of, a cylinder on the outer periphery of the lens holder 11A. Further, four rectangular driving coils for tracking 15a are secured to the outer periphery thereof. They are arranged so that one side of each of the driving coils for tracking 15a is located within a magnetic circuit including an inner and an outer core 13 and 18.
In the example indicated in FIG. 17, an objective lens 22 is mounted on a lens holder 21, which is rotatable around a journal 26 and movable in the direction of the journal 26 and at the same time a pair of rectangular-tube-shaped driving coils for focusing coil 25b are mounted at symmetrical positions, putting the journal 26 therebetween. Further, two rectangular driving coils for tracking coil 25a are mounted on the outer side surface of each of the coils 25b. They are arranged so that one side of each of the coils 25a is located within a magnetic circuit including an inner and an outer core 23 and 28 as will as a magnet 27.
In the example indicated in FIGS. 18 to 20, an objective lens 32 is mounted at the center of a rectangular pillar shaped lens holder 31 and at the same time a rectangular-tube-shaped driving coil for focusing 35b is mounted on each of the two side surfaces thereof, which are parallel to each other. Further, a driving coil for tracking 35a formed by bending a coil formed in the shape of a rectangle in a U-shape is mounted so as to enclose about a half of the outer side surface of each of the coils 35b, as indicated in FIG. 19. The lens holder 31 stated above is supported movably in the direction of the optical axis of the objective lens 32 and in the direction perpendicular to the first by means of a parallel spring 34. A magnetic circuit is constituted by a yoke 33 having two protrusions 33a, 33c on the two side surfaces, a central protrusion 33b and a magnet 38 secured to the inner side surface of each of the protrusions 33a, 33c as indicated in FIG. 20, and so constructed that the central protrusion 33b stated above is located at the center of the coil 35b so that the magnetic flux traverses two sides of each of the coils 35b, 35a.
In the example indicated in FIGS. 21 and 22, an objective lens 42 is mounted on a lens holder 41, which is rotatable around the journal 46 and movable along the journal 46, and at the same time a pair of driving coils for tracking 45a are mounted on both the sides thereof. Further, a cylindrical driving coil for focusing 45b is mounted under this coil 45a so as to enclose the journal 46. A magnetic circuit for tracking 43 is constituted by a yoke and a magnet disposed opposite to the coil 45a stated above and on the other hand a magnetic circuit for focusing 48 is constituted by a yoke and a magnet disposed opposite to the coil 45b stated above.
In FIGS. 23A to 23D and 23a to 23d, the utilization efficiency of the driving coil for focusing and the driving coil for tracking in the prior art objective lens driving device described above, i.e. the ratio of the length of the portion, on which the thrusting force acts effectively, to the total length of the coil, is indicating by hatching. FIGS. 23A and 23a correspond to FIG. 16; FIGS. 23B and 23b to FIG. 17; and FIGS. 23C and 23c to FIGS. 18 to 20. As it can be seen from these figures, according to the three prior art examples indicated in FIGS. 16 to 20, since the ratio of the portion, in which each of the coils generates effectively the driving force, is small, they have a problem that the utilization efficiency is low and the sensitivity is low. In addition, in these examples, since the magnet is used in common by winding the driving coils for focusing and for tracking, superposed on each other in the magnetic gap, at the first glance it seems that they have a tendency towards a higher efficiency. However, in reality, since the coils are wound, superposed on each other, the magnetic gap is enlarged and the magnetic flux density within the gap is lowered. Therefore, the size of the efficiency is cancelled and the superposed winding of the coils does not contribute largely to the rise of the efficiency.
Contrarily thereto, according to the prior art example indicated in FIGS. 21 and 22, since there are disposed separately magnetic circuits used exclusively for focusing and for tracking, respectively, as indicated in FIGS. 23D and 23d, the utilization efficiency of each of the driving coil is high and therefore a high performance objective lens driving device can be expected. However, since the magnetic circuits are used exclusively for focusing and for tracking separately, the cost therefor is high. Further, it is difficult to dispose a counter yoke on the lens holder side in the magnetic circuit for tracking. Therefore, in reality the magnetic flux density in the coil portion is low and the efficiency is not always high.
As described above, in an objective lens driving device in the optical pickup used in an optical disk device, etc. it is necessary to control the position of the objective lens with a high precision two-dimensionally in the focusing direction and the tracking direction. In order to effect this two-dimensional following servo, the structure for holding the objective lens at a predetermined neutral point is an important factor determining the performance of the optical pickup.
Structure for supporting the objective lens in prior art objective lens driving devices can be classified roughly into two categories with respect to the method, by which the objective lens is held at the predetermined neutral point, one of them using a metallic plate spring and in the other an elasticity holding damper such as a molded spring being combined with the supporting journal. The former can be used in common for positioning the lens and for holding the elasticity. However, it is apt to give rise to extraordinary resonance due to slight distortion of the plate spring or to be instable because of insufficiency of the braking property at the resonance and in addition it is difficult to locate the optical axis of the lens with a high precision. For the latter, although it is each to locate the optical axis with a high precision by means of the supporting journal, since most of the elastic members combined therewith are molded products made of rubber, resin, etc., it is disadvantageous in view of temperature characteristics or changes with the passage of time and at the same time, since the size of the spring is reduced together with the reduction of the size of the device, it has a problem that the region, where the characteristics are linear, becomes narrower and that the follow-up property is worsened for disks having a large surface oscillation or excentricity.
Consequently, in order to solve these problematical points, several propositions have been done, by which the objective lens is held at the neutral point by making it have a magnetic restoring force. Several examples thereof are those described in JP-Utility Model-A-58-179635, JP-Utility Model A-58-163908 and JP-A-62-141646.
In the objective lens driving device described above, in which the objective lens has an elastic restoring force, the region where the elastic restoring force is uniform is narrow, and in order to remedy it, improvement is necessary to use a plurality of magnetic pieces, etc. Further, holding of the neutral point by means of a magnetic piece is efficient for either one of the focusing direction and the tracking direction. Therefore, it has a problem that a pair of magnetic pieces are necessary and the construction is complicated, which raises the cost, if it is intended to have a neutral point in both the directions, i.e. focusing direction and tracking direction.